Semiconductor device

ABSTRACT

Semiconductor device comprising a source electrode, a drain electrode and a semiconducting layer consisting of a single or double 2-dimensional layer(s) made from one of the following materials: MoS 2 , MoSe 2 , WS 2 , WSe 2 , MoTe 2  or WTe 2 .

FIELD OF INVENTION

The present invention relates to semiconductor devices such as transistors or inverters. It more precisely relates to semiconductors comprising a part made of one or two 2-dimensional layers of MoS₂, MoSe₂, WS₂, WSe₂, MoTe₂, or WTe₂.

PRIOR ART

One of the basic building blocks of modern electronics is a field-effect transistor operating as a switch. The operation of FETs with dimensions lower than 50 nm are limited by short channel effects. One of them is the reduction of the height of the tunneling barrier that charge carriers need to overcome when the transistor is supposed to be in its off state, due to a reduction in effectiveness of electrostatic control exercised by the gate electrode. This results in high amounts of power dissipation when the transistor is expected to be off. The use of individual 2-dimensional semiconducting layers can reduce this power dissipation because the 2-dimensional geometry of the semiconducting layer results in improved electrostatic control of the gate electrode and reduction of short channel effects when compared to thick semiconducting layers or stacks of 2-dimensional layers of MoS₂, MoSe₂, WS₂, or WSe₂, MoTe₂, WTe₂. Smallest allowable channel length is proportional to square root of material thickness (L √{square root over (d)}), so by using a thinner material one can fabricate a transistor with a shorter channel. In the case of single-layer MoS₂, 0.65 nm thick, a transistor with a channel 1.5 nm long is still functional.

Another advantage in the use of one or two 2-dimensional layers in FETs lies in the fact that their band gap can be tuned either by a reduction of the number of layers where all the above-mentioned 2-dimensional layers have a direct band gap which is higher than the band gaps of stacks of related materials. This higher band gap leads to further reductions in power dissipation. The size of the band gap can also be tuned using a transverse electric field. Such tuning can result in more efficient transistor switching than in classical FETs.

In order to take a full advantage of the 2-dimensional material, it is advantageous to use a dielectric layer that encapsulates them and which can result in increased charge carrier mobility through physical mechanisms of charge carrier screening, phonon spectrum modification or induced strain. In the example of MoS₂, the use of a HfO₂ dielectric layer can increase the charge carrier mobility by a factor ˜1000.

The use of stacks of 2-dimensional layers of MoS₂, MoSe₂, WS₂, or WSe₂, MoTe₂, WTe₂ have been proposed for FETs for instance in US patent application US2007/181938A1.

FETs using such stacks of 2-dimensional layers are however characterized with several disadvantages such as a low electrostatic control, important power dissipation, absence of a direct band gap and a poor tunability.

There is therefore a need to improve those existing semiconductor devices.

GENERAL DESCRIPTION OF INVENTION

The invention concerns a semiconductor device comprising a source electrode, a drain electrode and a semiconducting layer consisting of a single or double 2-dimensional layer(s) made from one of the following materials: MoS₂, MoSe₂, WS₂, WSe₂, MoTe₂ or WTe₂.

Preferred embodiments of the invention are defined in the dependent claims.

Replacing a stack by only one or two 2-dimensional layers of MoS₂, MoSe₂, WS₂, or WSe₂, MoTe₂, WTe₂ provides an enhanced electrostatic control, low power dissipation, direct band gap and tunability. It was shown in Example 1 that a transistor based on a single layer of MoS₂ has a room-temperature on/off ratio of 10⁸ and an off-state current of 25fA/μm. This is because the single 2-dimensional layer has a higher band gap than stacks of 2-dimensional layers and because the gate can more efficiently control conduction due to small thickness of the semiconducting material and result in smaller power consumption. Stacks based on dichalcogenide materials described in patent application US2007/181938A1 have a room-temperature on/off ratio on the order of 10 because of the relatively high thickness of the semiconducting material. Such low on/off ratio implies high power consumption in the off state of the transistor. Using a double 2-dimensional layer retains the advantage of a thin material with the additional benefit that the band gap can be tuned with the application of the external electric field. These advantages are no longer present in structures containing 3 or more 2-dimensional layers.

The advantages of using single or double 2-dimensional layers over using stacks of 2-dimensional layers are not obvious to those familiar with the art. A previously shown example (Novoselov et al., Two-dimensional atomic crystals, PNAS 102, 10451 (2005).) does not exhibit turn-off but only shows a linear dependence of conductivity on bottom gate voltage. Such devices cannot be used as field effect transistors. Mak et al. (Mak et al., Atomically Thin MoS₂: A New Direct-Gap Semiconductor, Phys. Rev. Lett. 105, 136805 (2010)) also shows a device that does not turn-off, shows a current modulation by a factor of 100, requiring 160 volts change on the gate electrode which is not practical in devices. All these examples of prior art do invite the use of single 2-dimensional layers in transistor and do not show that using single 2-dimensional layers has advantages over using stacks of 2-dimensional layers.

FIG. 1A is a schematic non-limitative representation of a single-layer MoS₂ transistor according to the invention.

FIG. 1B illustrates the structure of a single two-dimensional layer of MoS₂.

Alternatively the single MoS₂ layer may be replaced a layer made of MoSe₂, WS₂, or WSe₂, MoTe₂, WTe₂. Two 2-dimensional layers may also be used.

Source, drain and top gate electrodes may be made of any electrical conductor, including graphene.

The dielectric layer may be made of any electrically insulating material, including boron-nitride and single-layer boron nitride.

In one embodiment of the invention a bottom dielectric layer may be used, such a layer being made of any electrically insulating material, including boron-nitride and single-layer boron nitride.

The bottom dielectric layer may be omitted if using an electrically insulating substrate.

Substrate may be made of any suitable flat material, solid or flexible; at least: glass, sapphire, kapton, polyimide.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be better understood below, in particular with the use of some non-limiting examples.

The semiconductor device according to the invention may be manufactured by one of the following processes:

-   -   a) Liquid phase exfoliation—this is the method developed by the         Oxford-Trinity college collaboration (Coleman et al.,         Two-Dimensional Nanosheets Produced by Liquid Exfoliation of         Layered Materials, Science 331, 568 (2011). It consists of         suspending particles of the material in organic solvents,         agitating using solvents using ultrasound and purifying using         centrifugation. These solvents can then be sprayed over surfaces         or deposited on substrates using the Langmuir-Blodgett technique         (Divigalpitiya et al., Thin oriented films of molybdenum         disulphide, Thin Solid Films 186, 177 (1990)). This may be the         method of choice for applications in cheap, bendable         electronics.     -   b) Molecular beam epitaxy—elements (Mo and Se for example) are         simultaneously evaporated in ultrahigh vacuum onto heated         substrates. High quality, continuous films can be grown on large         areas (cm×cm or larger). (Ueno et al., Epitaxial-Growth of         Transition-Metal Dichalcogenides on Cleaved Faces of Mica,         Journal of Vacuum Science & Technology A 8, 68 (1990)).     -   c) Chemical vapour transport—either elements (Mo, Se, Te, S, W)         or powders (for example MoSe₂) are sublimated in closed ampoules         with a warmer and a colder region. Crystals deposit and grow in         the colder part of the tube (Enomoto et al., Van der Waals         growth of thin TaS₂ on layered substrates by chemical vapor         transport technique, Jpn. J. Appl. Phys. 43, L123 (2004)).     -   d) Chemical vapour deposition—mixture of gasses flow past a         heated catalyst surface. A chemical reaction occurs on the         surface, resulting in film growth (Boscher et al., Atmospheric         Pressure CVD of Molybdenum Diselenide Films on Glass, Chemical         Vapor Deposition 12, 692 (2006)).

In addition to the basic structure shown on FIG. 1A the semiconductor device according to the present invention may be made according to other geometries.

Some non-limitative examples of such geometries are presented in FIGS. 2A to 2I wherein:

FIG. 2A shows a top gate electrode which extends over the other electrodes.

The device of FIG. 2B has no top gate electrode or top oxide. It can be used as a chemical, biological or pH sensor.

In the device of FIG. 2C the top gate electrode is embedded in gate oxide.

The device of FIG. 2D comprises two transistors connected in series (logic inverter).

The device of FIG. 2E comprises three transistors connected in series (logic NOR).

In the device of FIG. 2F the gate oxide is replaced with a ferroelectric material (non-volatile memory).

The device of FIG. 2G shows an additional buried gate electrode, a better electrostatic control and tenability of the band gap in the case where two 2-dimensional layers are used as a semiconducting material.

The device of FIG. 2H contains a buried gate electrode, has no top gate or top oxide and can therefore be used as a sensor.

The device of FIG. 2I has two transistors in series and a junction with n and p-type doping. It can be used as LED.

FIG. 3 shows the structure and AFM imaging of monolayer MoS₂. a Three-dimensional representation of the structure of MoS2. Single layers, 6.5 Å thick can be extracted using scotch-tape based micromechanical cleavage. b Atomic force microscope image of a single layer of MoS2 deposited on Si substrate with a 270 nm thick oxide layer. c Cross-section plot along the red line in part b.

FIG. 4 shows the fabrication of MoS₂ monolayer transistors. a Optical image of a single layer of MoS₂ with a thickness of 6.5 Å deposited on top of a Si substrate with a 270 nm thick SiO₂ layer. b Optical image of a device based on the flake shown on figure a. The device consists of two field effect transistors connected in series and defined by three Au leads that serve as source and drain electrodes for the two transistors. Monolayer MoS₂ is covered by 30 nm of ALD deposited HfO₂ that acts both as a gate dielectric and a mobility booster. The scale bars in a and b are 10 μm long. c Three-dimensional schematic view of one of the transistors shown on figure b.

FIG. 5 shows characterization of MoS₂ monolayer transistors. a Cross-sectional view of the structure of monolayer MoS₂ FET together with electrical connections used to characterize the device. Single layer of MoS₂ 6.5 Å thick is deposited on degenerately doped Si substrate with 270 nm thick SiO₂. The substrate acts a back gate. One of the gold electrodes acts as drain while the other, source electrode is grounded. The monolayer is separated from the top gate by 30 nm of ALD-grown HfO₂. The top gate width for device is 4 μm while the top gate length, source-gate and gate-drain spacing is 500 nm. b Room temperature transfer characteristic for the FET with 10 mV applied bias voltage V_(ds). Back gate voltage V_(bg) is applied to the substrate and the top gate is disconnected. The inset shows an I_(ds)-V_(ds) curve acquired for V_(bg) values of 0.1 and 5 V.

FIG. 6 shows local gate control of the MoS₂ monolayer transistor. a I_(ds)-V_(tg) curve recorded for a bias voltages ranging from 10 mV to 500 mV. Measurements are performed at room temperature with the back gate grounded. Top gate width is 4 μm while the top gate length is 500 nm. The device can be completely turned off by changing the top gate bias from −2 to −4 V. For V_(ds)=10 mV, the I_(On)/I_(Off) ratio is >1×10⁶ . For V_(ds)=500 mV, the I_(On)/I_(Off) ratio is >1×10 ⁸ in the measured range while the subthreshold swing S=74 mV/dec. Top and bottom gate leakage is negligible (Fig. S3 in supplementary materials). Inset shows I_(ds)-V_(tg) for values of V_(bg)=−10,−5,0,5 and 10 V. b I_(ds)-V_(ds) curves recorded for different values of the top gate voltage V_(tg). The linear dependence of the current on bias voltage for small voltages indicates that the Au contacts are ohmic.

FIG. 7 shows an integrated circuit based on single-layer MoS₂ (not to scale). (a) Single-layer MoS₂ is deposited on top of a Si chip covered with 270 nm thick SiO₂. The integrated circuit is composed of two transistors defined by a neighboring pair of leads and controlled by local gates with HfO₂ gate dielectric. (b) Cross-sectional view of the structure of a monolayer MoS₂ integrated circuit together with electrical connections used to characterize the device. One of the gold electrodes acts as drain while the other, source electrode is grounded. The monolayer is separated from the top gate by 30 nm of ALD-grown HfO₂. The top gate width for device is 4.7 μm, top gate length is 1.3 μm and lead spacing is 1.6 μm. The substrate can act as a back gate but is kept grounded during the measurement.

FIG. 8 shows electrical characterization of the integrated circuit based on monolayer MoS₂. (a) Optical image of a monolayer MoS₂ deposited on top of a Si substrate with a 270 nm thick SiO₂ layer. (b) Integrated circuit based on the flake shown in (a). The device consists of three Au electrical leads that can act as source, drain and output terminals and two local gates. The scale bars in (a) and (b) are 10 μm long. (c) Drain-source current I_(ds) through the MoS₂ monolayer transistor on the left side of the integrated circuit shown on b, measured as a function of the top gate voltage V_(tg). The MoS₂ transistor shows gating response typical of FETs with n-type conducting channels. Inset shows the drain-source current I_(ds) as a function of back gate voltage V_(bg) for drain-source voltage V_(ds) values of 100 mV, 200 mV and 500 mV. (d) Drain-source current I_(ds) as a function of drain-source voltage for different values of V_(tg). The current through the device changes by over six orders of magnitude when the top gate voltage is swept in the −4V to +4V range.

FIG. 9 shows characteristics of the integrated MoS₂ inverter. (a) Output voltage as a function of the input voltage. Schematic drawing of the electronic circuit and the truth table for the NOT logic operation (Inset). (b) The dependence of the inverter gain (negative value of dV_(out)/dV_(t).) on the input voltage. The maximal voltage gain above 4 indicates that our inverter is suitable for integration in arrays of logic devices.

FIG. 10 shows the demonstration of a NOR-gate logic circuit based on single-layer MoS₂ transistors. The circuit is formed by connecting two monolayer MoS₂ transistors in parallel and using an external 1 MOhm resistor as a load (left inset). The output voltage V_(out) is shown for four different combinations on input states (1,0), (1,1), (0,1) and (0,0). The output is at the high state only if both inputs are in the low state (truth table in the inset). All logic operations can be expressed as combinations of NOR operations.

Two detailed non-limitative examples according to the present invention are presented in the following chapters.

EXAMPLE 1 Transistor Based on a Single 2-Dimensional Layer of MoS₂

Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, ^(1,2) both because of its rich physics³⁻⁵ and its high mobility.⁶ However, pristine graphene does not have a band gap, which is required for many applications including transistors,⁷ and engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films⁸⁻¹³ or requires high voltages.^(14,15) While single layers of MoS₂ have a large intrinsic bandgap of 1.8 eV¹⁶, previously reported mobilities in the 0.5-3 cm²/Vs range¹⁷ are too low for practical devices. Here we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS₂ mobility of at least 200 cm²/Vs, similar to that of graphene nanoribbons, and demonstrate transistors with room temperature current on/off ratios of 10⁸ and ultralow stand-by power dissipation. Because monolayer MoS₂ has a direct bandgap^(16,18) it can be used to construct inter-band tunnel FETs¹⁹, which offer lower power consumption than classical transistors. Monolayer MoS₂ could also complement graphene in applications that require thin transparent semiconductors—for example in optoelectronics and energy harvesting.

MoS₂ is a typical example from the layered transition metal dichalcogenide family of materials. Crystals of MoS₂ are composed of vertically stacked, weakly interacting layers held together via van der Waals interaction, FIG. 3 a. Single layers, 6.5 Å thick (FIGS. 3 b and 3 c), can be extracted by means of scotch tape^(17,23) or lithium-based intercalation.^(24,25) Large-area thin films can also be prepared using MoS₂ suspensions. Bulk MoS₂ is semiconducting with an indirect band gap of 1.2 eV,²⁶ while single-layer MoS₂ is a direct gap semiconductor^(16,18) with a band gap of 1.8 eV.¹⁶ MoS₂ nanotubes²⁷ and nanowires²⁸ also show the influence of quantum-mechanical confinement in their electronic and optical properties. Other features that could make MoS₂ interesting for nanoelectronic applications include the absence of dangling bonds and thermal stability up to 1100° C.

Single-layer MoS₂ could also be interesting as a semiconducting analogue of graphene that does not have a band gap in its pristine form. Band gaps up to 400 meV have been introduced by quantum mechanical confinement in patterned²⁰ or exfoliated graphene nanoribbons⁹ but always at the price of significant mobility reduction (200 cm²/Vs for a 150 meV band gap),^(9,10) loss of coherence¹¹ or increased off-state currents due to edge roughness.¹² Band gaps were also induced by applying a perpendicular electric field in bilayer graphene,^(14,21) but the highest reported optical gap here is 250 meV, requiring application of voltage exceeding 100V.¹⁴ This makes it very difficult to build logic circuits based on graphene that would operate at room temperature with low stand-by power dissipation. In fact, any potential replacement of silicon in CMOS-like digital logic devices is desired to have a current on/off ratio⁷ I_(on)/I_(off) between 10⁴ and 10⁷ and a band gap exceeding 400 meV.²²

The starting point for the fabrication of our transistors is scotch-tape based micromechanical exfoliation^(1,17) of single-layer MoS₂. MoS₂ monolayers are transferred to degenerately doped Si substrates covered with 270 nm thick SiO₂, FIG. 4 a. We have previously found that this oxide thickness is optimal for optical detection of single-layer MoS₂ and established the correlation between contrast and thickness as measured by atomic force microscopy.²⁹ Electrical contacts are first fabricated using electron beam lithography followed by deposition of 50 nm thick Au electrodes. The device is then annealed at 200° C. in order to remove resist residue³⁰ and decrease contact resistance (please refer to the supplementary material for more details). At this point our single-layer devices show typical mobility in the 0.1-10 cm²V/s range, similar to previously reported values for single-layers¹⁷ and thin crystals containing more than 10 layers of MoS₂.³¹ This is lower than the previously reported phonon-scattering limited room temperature mobility in the 200-500 cm²/Vs range for bulk MoS₂.³² Encouraged by recent theoretical predictions of mobility improvement by dielectric screening³³ and its successful application to graphene,³⁴ we proceed by atomic layer deposition of 30 nm HfO₂ as a high-x gate dielectric for the local top gate and mobility booster in order to realize the full potential of single-layer MoS₂. We chose HfO₂ because of its high dielectric constant of 25, band gap of 5.7 eV and the fact that it is commonly used as a gate dielectric both by the research community and major microprocessor manufacturers.^(35,36) The resulting structure, composed of two field-effect transistors connected in series is shown on FIG. 4 b with the schematic depiction of the device shown on FIG. 4 c. The top gate width for our device is 4 μm while the top gate length, source-gate and gate-drain spacing is 500 nm.

We perform electrical characterization of our device at room temperature using a semiconductor parameter analyzer and shielded probe station with voltage sources connected in the configuration depicted on FIG. 5 a. We first characterize our MoS₂ transistors with 6.5 Å thick conductive channels by applying a drain-source bias V_(ds) to a pair of Au electrodes and gate voltage V_(bg) to the degenerately doped Si substrate while leaving the top gate electrically floating.³⁷ The gating characteristics of the left-most transistor shown on FIG. 4 b is presented on FIG. 5 b and is typical of FET devices with an n-type channel. We concentrate on this device in the remainder of the manuscript. Characterization details for other devices and fabrication batches are available in the supplementary material. All the MoS₂ transistors we have fabricated, regardless of the number of layers or contacting material, show behavior typical of FET devices with n-type channels. Repeated V_(bg) sweeps on the same device do not show significant variation, while keeping all the voltages constant results in constant I_(ds), indicating that the top gate is not likely to accumulate charge during measurements. We estimate that a constant surface charge n≈4.6×10¹² cm⁻² trapped on the top gate would shift the threshold voltage by ˜1V but not change the slope of the I_(ds)-V_(bg) curve on FIG. 5 b used for estimating channel mobility. The source current versus source bias characteristics, FIG. 5 b inset, is linear in the ±50 mV range of voltages, indicating that our Au contacts are ohmic.

The on-resistance of our transistor is 27 kΩ for V_(ds)=10 mV and V_(bg)=10 V with the gate width of 4 μm and bottom gate length of 1.5 μm. We have noticed that the device resistance can increase during storage at ambient conditions for a period of two months. This could be attributed to absorption of oxygen and/or water from the environment and could be mitigated by device encapsulation.

From the data presented on FIG. 5 b we can extract the low-field field effect mobility of ˜217 cm²/Vs using the expression μ=[dI_(ds)/dV_(bs)]×[Ll(WC_(i)V_(ds))] where L=1.5 μm is the channel length, W=4 μm channel width and C_(i)=1.3×10⁻⁴ F/m² is the capacitance between the channel and the back gate per unit area (C_(i)=ε₀ε_(r)/d; ε_(r)=3.9; d=270 nm). Note that this value represents the lower limit because of contact resistance. As our device displays ohmic I_(ds)-V_(ds) behavior, FIG. 5 b inset, we exclude the possibility that our field-effect behavior is dominated by Schottky barriers at the contacts.

Even though the room-temperature value of phonon-scattering limited³² mobility for bulk MoS₂ is in the 200-500 cm²/Vs range, exfoliation of single layers onto SiO₂ results in a decrease of mobility down to the 0.1-10 cm²V/s range. The improvement of the mobility with the deposition of a high-κ dielectric could be due to suppression of Coulomb scattering due to the high-κ dielectric environment³³ and modification of phonon dispersion³⁸ in MoS₂ monolayers. Extensive future theoretical work including calculation of phonon dispersion relation in single-layer MoS₂, calculation of scattering rates on phonons and charge impurities would be needed to provide a complete picture.

Before we compare the value of mobility in our case with the mobility of graphene or thin-film Si we should note that semiconductors such as carbon nanotubes or graphene nanoribbons mostly follow the general trend of decreasing mobility with increasing band gap.²² Even though graphene has a high room temperature mobility of 120000 cm²/Vs, this value relates to large-area, gapless graphene.⁶ On the other hand, measurements on 10 nm wide graphene nanoribbons with Eg˜400 mV indicate mobility lower than 200 cm²/Vs,⁹ in good agreement with theoretical models that predict decreased mobility in small-width GNR due to electron-phonon scattering.¹³ This is comparable to mobility of 250 cm²/Vs found in 2 nm thin strained Si films.⁴² Our MoS₂ monolayer has similar mobility but higher band gap than graphene nanoribbons⁹ and a smaller thickness than thinnest Si films fabricated to date.⁴²

One of the crucial requirements for building integrated circuits based on single layers of MoS₂ is the ability to control charge density in a local manner, independently of a global back gate. We can do this by applying a voltage V_(tg) to the top gate, separated from the monolayer MoS₂ by 30 nm of HfO₂, FIG. 5 a, while keeping the substrate grounded. The corresponding transfer characteristic is shown on FIG. 6 a. For a bias of 10 mV we observe an on current of 150 nA (37 nA/μ), current on/off ratio I_(on)/I_(off)>10⁶ for the ±4 V range of V_(tg), an off state current that is smaller than 100 fA (25fA/μm) and gate leakage lower than 2 pA/μm². The observed current variation for different values of V_(tg) indicates that the field-effect behavior of our transistor is dominated by the MoS₂ channel and not the contacts.

At the bias voltage V_(ds)=500 mV, the maximal measured on current is 10 μA (2.5 μA/μm), with I_(on)/I_(off) higher than 10⁸ for the ±4 V range of V_(tg). The device transconductance defined as g_(m)=dI_(ds)/dV_(tg) at V_(ds)=500 mV is ˜4 μS (1 μS/μm), similar to values obtained for high performance CdS nanoribbon array transistors (2.5 μS/μm at V_(ds)=1 V).⁴³ High-performance top-gated graphene transistors can have normalized transconductane values as high as 1.27 mS/μm.⁴⁴ The large degree of current control in our device is also clearly illustrated on FIG. 6 b. where we plot the drain-source current versus drain-source bias for different values of voltage applied to the local gate. From the channel current dependence on top-gate voltage, we deduce a subthreshold slope for the transition between the on and off states of 74 mV/dec for a bias V_(ds)=500 mV. Being a direct gap semiconductor, single layers of MoS₂ offer the intriguing possibility for the realization of an interband tunnel FETs which is characterized by turn-on sharper than the theoretical limit of 60 mV/dec for classical transistors and consequently smaller power dissipation. This feat has remained difficult in case of silicon, an indirect gap semiconductor, because interband transitions there require phonons and recombination centers.

To summarize, we have realized a field-effect transistor with a single, two-dimensional layer of a semiconductor MoS₂ as a conductive channel and hafnium dioxide as a gate insulator. The conductive channel in our device is only 6.5 Å thick. Our transistor exhibits a room temperature current on/off ratio exceeding 1×10⁸ and mobility of ˜200 cm²/Vs, comparable to mobility achieved in thin silicon films⁴² or graphene nanoribbons.⁹ Such a transistor could form the backbone of future electronics based on layered materials where MoS₂ transistors could be fabricated on insulating boron-nitride substrates.⁴⁰ Our results provide an important step towards the realization of electronics and low stand-by power integrated circuits based on two-dimensional materials. Being a thin, transparent semiconducting material, MoS₂ monolayers also present a wealth of new opportunities in areas that include mesoscopic physics, optoelectronics and energy harvesting. With the possibility to fabricate large areas circuits using solution-based processing, our finding could be important for producing electronic devices that could combine the ease of processing associated with organic conductors with performance figures commonly associated with Si-based electronics.

Materials and Methods

Single layers of MoS₂ are exfoliated from commercially available crystals of molybdenite (SPI Supplies Brand Moly Disulfide) using the scotch-tape micromechanical cleavage technique method pioneered for the production of graphene.¹ AFM imaging is performed using the Asylum Research Cypher AFM. After Au contact deposition, devices are annealed in 100 sccm of Ar and 10 sccm H₂ flow at 200 C for 2 h.³⁰ ALD is performed in a home-built reactor using a reaction of H₂O with tetrakis(dimethylamido)hafnium (Sigma Aldrich). Electrical characterization is carried out using Agilent E5270B parameter analyzer and a home-built shielded probe station with micromanipulated probes.

REFERENCES TO EXAMPLE 1

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Graphene     Field-Effect Transistors with High On/Off Current Ratio and Large     Transport Band Gap at Room Temperature. Nano Lett. 10, 715-718,     (2010). -   22 Schwierz, F. Graphene transistors. Nature Nanotech. 5, 487-496,     (2010). -   23 Frindt, R. F. Single Crystals of MoS2 Several Molecular Layers     Thick. J. App. Phys. 37, 1928-1929, (1966). -   24 Joensen, P., Frindt, R. F. & Morrison, S. R. Single-Layer MoS2.     Mat. Res. Bull. 21, 457-461, (1986). -   25 Schumacher, A., Scandella, L., Kruse, N. & Prins, R. Single-layer     MoS2 on mica: studies by means of scanning force microscopy. Surf.     Sci. Lett. 289, L595-L598, (1993). -   26 Kam, K. K. & Parkinson, B. A. Detailed photocurrent spectroscopy     of the semiconducting group VIB transition metal dichalcogenides. J.     Phys. Chem. 86, 463-467, (1982). -   27 Feldman, Y., Wasserman, E., Srolovitz, D. J. & Tenne, R.     High-Rate, Gas-phase grwth of MoS2 nested inorganic fullerenes and     nanotubes. 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EXAMPLE 2 Integrated Circuits, Inverters and Logic Operations Based on a Single 2-Dimensional Layer of MoS₂

Logic circuits and the ability to amplify electrical signals form the functional back-bone of electronics along with the possibility to integrate multiple elements on the same chip. Here, we demonstrate that single-layer MoS₂, a two-dimensional semiconductor with a direct band gap of 1.8 eV (ref 13), has the capability to amplify signals and perform basic logic operations in simple integrated circuits composed of two MoS₂ transistors.³ Our integrated circuit is composed of two n-type transistors realized on the same two-dimensional crystal of monolayer MoS₂, as schematically depicted on FIG. 7( a) and (b).

Single-layer MoS₂ is a typical two-dimensional semiconductor from the layered transition metal dichalcogenide family. Single layers, 6.5 Å thick, can be extracted from bulk crystals using the micromechanical cleavage technique commonly associated with the production of graphene,^(1, 14) lithium-based intercalation¹⁵⁻¹⁶ or liquid phase exfoliation¹⁷ and used as ready-made blocks for electronics.³ Decreasing the number of layers in mesoscopic MoS₂ structures leads to a transformation from an indirect band gap semiconductor with a band gap of 1.2 eV (ref 18) into a direct gap semiconductor,^(13. 19-21) with a band gap of 1.8 eV (ref 13) due to quantum confinement.²¹ Being an ultrathin direct gap semiconductor, single-layer MoS₂ is very interesting as a material complementary to graphene which does not have a band gap in its pristine form. This makes it very difficult to fabricate logic circuits²²⁻²³ or amplifiers²⁴ that would operate at room temperature with a voltage gain>1 which is necessary for incorporating such structures in electronic circuits. The presence of band-gap in single-layer MoS₂ on the other hand allows the realization of field-effect transistors with room-temperature on/off ratios that can exceed 10⁸ (ref. 3) which makes them interesting for building logic devices with low power dissipation. Recent simulations also predict that short channel single-layer MoS₂ devices could have higher on-current density than those based on Si,²⁵ ²⁶ current on/off ratio higher than 10¹⁰, high degree of immunity to short channel effects^(2, 27) and abrupt switching.²⁶ All these properties show that MoS₂ could be an interesting material for future applications in nanoelectronics.

We begin our integrated circuit fabrication by exfoliating single-layer MoS₂, FIG. 8( b), from bulk crystals using scotch-tape based micromechanical exfoliation,^(1, 4) commonly used for the production of graphene. Monolayers of MoS₂ are deposited on degenerately doped Si substrates covered with 270 nm thick SiO₂, resulting in optimal contrast for optical detection of single-layers.²⁸ Three electrical leads are first fabricated using standard electron-beam lithography, followed by deposition of 90 nm thick Au electrodes and annealing in Ar/H₂ mixture²⁹ in order to remove resist residue and decrease contact resistance. The device is then covered by atomic layer deposition of 30 nm HfO₂, a high-κ material commonly used as a gate dielectric.³⁰⁻³¹ Finally, local top gates are deposited in the final round of e-beam lithography and metal deposition resulting in an integrated circuit such as the one shown on FIG. 8( b), composed of two single-layer transistors connected in series. The channel width of the transistors in our integrated circuit is 4.2 μm, lead spacing is 1.6 μm and top gate length is 1.3 μm.

Both transistors in our integrated circuit can be independently controlled by applying a voltage V_(tg) to the corresponding top gate which is one of the crucial requirements for constructing integrated circuits composed of multiple transistors realized on the same substrate. We characterize both transistors in our integrated circuit at room temperature by connecting a pair of neighboring leads to the source and ground terminals of a semiconductor parameter analyzer and a voltage V_(tg) to the corresponding top gate. The substrate is grounded throughout the measurements. The transfer characteristic for the transistor on the right side of the integrated circuit is shown on FIG. 8( c), and is typical of n-type field-effect transistors. By changing the top gate voltage V_(tg) from −4V to +4V we can modify the current through the device over several orders of magnitude thanks to the high current on/off ratio of our transistor, higher than 10⁶ in this range of top-gate voltage V_(tg). The on-resistance is 24 kΩ for V_(ds)=100 mV and V_(tg)=4 V. The linear and symmetric I_(ds) vs V_(ds) characteristics shown on FIG. 8( d) indicates that the contacts are ohmic. From back-gating characteristics, shown in the inset of FIG. 8( c), we estimate the two-contact low-field field effect mobility of ˜320 cm²/Vs. At the bias voltage V_(ds)=500 mV, the maximal measured on current is 22 μA (4.6 μA/μm), with I_(on)/I_(off) higher than 10⁶ for the ±4 V range of V_(tg) and an I_(off)˜400 μA/μm.

The device transconductance defined as g_(m)=dI_(ds)/dV_(tg) is 12 μS (2.6 μS/μm) for V_(ds)=500 mV and is comparable to CdS nanoribbon array transistors (2.5 μS/μm at V_(ds)=1 V).³² High-performance top-gated graphene transistors can have normalized transconductance values³³ as high as 1.27 mS/μm while carbon nanotubes can reach transconductance of 2.3 mS/μm.³⁴ We expect that by lowering the channel length will reduce the number of scattering centers and increase the on current in MoS₂-based transistors. Theoretical models predict that MoS₂ transistors with a gate length of 15 nm would operate in the ballistic regime²⁵⁻²⁶ with a maximum ON current as high as 1.6 mA/μm and a transconductance of 4 mS/um, both for V_(g)=0.6 V and a bias V_(ds)=0.5 V.²⁶

The efficient channel switching for small voltages exhibited by our device is also clearly illustrated on Fig. (d) where we show the source-drain current I_(ds) dependence on source-drain voltage V_(ds) for different values of top gate voltage V_(tg). This large degree of control at room-temperature is necessary for realizing logic operations with large voltage gain. For development of logic circuits based on new materials, a voltage gain >1 is necessary so that the output of one logic gate could be used to drive the input of the next gate without the need for signal restoration.

We proceed by demonstrating that our single-layer MoS₂ integrated circuit can operate as the most basic logic gate: a logic inverter, capable of converting a logical 0 (low input voltage) into logical 1 (high output voltage). We connect the middle lead to one of the local gates in a configuration depicted in the inset of FIG. 9( a), commonly used in logic circuits based on only one type (n or p) of transistors. In this configuration, the “lower” transistor acts as a switch while the “upper” one acts as an active load. Input voltage V_(in) is applied to the local gate of the switch transistor while the supply voltage V_(dd)=2V is applied to the drain electrode of the load transistor. The output voltage and transfer characteristic of the inverter as a function of the input voltage V_(in) is shown on FIG. 9( a).

For input voltages corresponding to logic 0, the switch transistor has a higher resistance than the load transistor and is effectively turned off. This results in a constant voltage at the output terminal which is close to the supply voltage of V_(dd)=2V applied to the load drain electrode. By increasing the input voltage above −1V, the lower FET becomes more conductive and the output voltage V_(out) is now in the low range.

In the input voltage range of ±0.3 V, the output of the inverter is changing faster than the input, indicating that our device is capable of amplifying signals. The voltage gain defined as the negative of dV_(out)/dV_(in) and plotted on FIG. 9( b) is higher than 4. For successful implementation of digital logic in electronic circuits based on any new nanomaterial, a voltage gain>1 is needed so that the output of one inverter could drive the input of the next inverter in the cascade. Our inverter has a voltage gain higher than 4 and is therefore suitable for integration in arrays of logic gates. This could involve level shifters because the input and output logic levels are not the same. Increasing the threshold voltage of MoS₂ transistors using for example substrate functionalization could also bring input and output voltages to the same level.

We note that to the best of our knowledge the maximal voltage gain for graphene-based inverters²² demonstrated so far is 2-7 but at the temperature of 80K,^(23,35) due to the low bandgap (<100 meV) in bilayer graphene which prohibits the use of such devices at room temperature.

Our monolayer MoS₂ transistors can also be used to perform logic operations involving two operands. By connecting two transistors in parallel and using an external resistor as load, we can construct a NOR gate, shown on the inset of FIG. 10, where we show the output voltage for all the possible input states of the NOR gate. When either one or both of the transistors are in the “on” state (corresponding to V_(in)=2V), the output is ˜0V, corresponding to logical 0. Only when both transistors are in the “off” state, does the output become logical 1 (V_(out)≈V_(dd)=2V). NOR operation forms a functionally complete set of binary operations: every possibly logic operation (AND, OR, NAND etc.) can be realized using a network of NOR gates.

CONCLUSIONS

We have demonstrated here that single-layer MoS₂, a new two-dimensional semiconductor, can be used as the material basis for fabrication of integrated circuits and for performing logic operations with room-temperature characteristic suitable for integration. Our work represents the critical first step in the implementation of digital logic in two-dimensional materials at room temperature. Together with the possibility of large-scale liquid-based processing of MoS₂ and related 2D materials,¹⁷ our finding could open the way to using MoS₂ for applications in flexible electronics. Single-layer MoS₂ also has advantages over conventional silicon: it is thinner than state of the art silicon films that are 2 nm thick³⁶ and has a smaller dielectric constant (ε=7, ref. 37) than silicon (ε=11.9), implying that using single-layer MoS₂ could reduce short channel effects²⁶ and result in smaller and less power-hungry transistors than those based on silicon technology. Several difficulties however need to be solved before MoS₂ could become a mainstream electronic material for the semiconductor industry. A method for large-scale growth of continuous monolayers of MoS₂ or a similar 2D semiconductor will be needed to fabricate more complex integrated circuits with a large number of elements.

Materials and Methods

Single layers of MoS₂ are exfoliated from commercially available crystals of molybdenite (SPI Supplies Brand Moly Disulfide) using the scotch-tape micromechanical cleavage technique method pioneered for the production of graphene. AFM imaging is performed using the Asylum Research Cypher AFM. After Au contact deposition, devices are annealed in 100 sccm of Ar and 10 sccm H₂ flow at 200 C for 2 h.²⁹ ALD is performed in a commercially available system (Beneq) using a reaction of H₂O with tetrakis(ethyl-methylamido)hafnium. Electrical characterization is carried out using National Instruments DAQ cards and a home-built shielded probe station with micromanipulated probes.

REFERENCES TO EXAMPLE 2

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1. Semiconductor device comprising a source electrode, a drain electrode and a semiconducting layer consisting of a single or double 2-dimensional layer(s) made from one of the following materials: MoS₂, MoSe₂, WS₂, WSe₂, MoTe₂ or WTe₂.
 2. Semiconductor device according to claim 1 furthermore comprising a gate electrode and a dielectric layer placed between said semiconducting layer and the gate electrode, said dielectric layer being configured to protect the semiconducting layer from the environment and increase its mobility.
 3. Semiconductor device according to claim 2 wherein said gate electrode is extending over said source electrode and said drain electrode.
 4. Semiconductor device according to claim 2 wherein said gate electrode is embedded within said dielectric layer.
 5. Semiconductor device according to claim 1 devoid of a top gate electrode and a top dielectric layer.
 6. Sensor based on a semiconductor device according to claim
 5. 7. Semiconductor device according to claim 2 comprising a second gate electrode.
 8. Semiconductor device according to claim 7 wherein said second gate electrode is embedded within an oxide layer.
 9. Semiconductor device according to claim 1, wherein said semiconducting layer is consisting of a single 2-dimensional layer.
 10. Semiconductor device according to claim 1, wherein said semiconducting layer is a nanosized layer.
 11. Semiconductor device according to claim 10 wherein said semiconducting layer is between 0.5 and 1.5 nm thick.
 12. Semiconductor device according to claim 1, wherein at least one of said electrodes is made of graphene.
 13. Semiconductor device according to claim 1, wherein said dielectric layer is made of HfO₂.
 14. Transistor comprising a semiconductor device according to claim
 1. 15. Digital inverter comprising a semiconductor device according to claim
 1. 